Epstein Barr virus (EBV) immunovirology was investigated as a marker for diagnosis/prognosis in nasopharyngeal carcinoma (NPC) patients, their families, and individuals from ethnic populations in Malaysia known to be at higher risk of developing NPC by comparing IgA antibody to EBV virus capsid antigen (VCA) and lymphocyte stimulation by EBV antigens. All 75 VCA-IgA antibody-positive sera from NPC patients, regardless of age, sex and race (IF titers 10-640), blocked lymphocyte stimulation from NPC patients (20). Those sera which did not contain VCA-IgA antibody (less than 1:5) did not block lymphocyte stimulation and these patients were found either to have been in remission for at least a year or had no evidence of tumor. Of 25 family members, two had VCA-IgA titers of 5 and 10 and their sera contained lymphocyte stimulation inhibition (LSI). Even though VCA-IgA in one remained between 10-20 over 1.5 years, LSI titers increased after a year and NPC was diagnosed. The peak incidence of NPC in Malaysian Cantonese Chinese was 40-49 years of age for VCA-IgA and was found in all three histological types. In Malay Kadazans, higher incidences were found in young as well as old age groups and were different than those of the Chinese. Human cord blood mononuclear cells were shown to possess receptors for Herpesvirus saimiri (HVS). The infected cells were identified as T cells, IL2 dependent and HVS virus- and antigen-positive. Three different human fibroblast cell lines were tested for HVS replication/transformation. Only one was infectable with HVS regardless of the strain used. Infected cells were virus positive. A rabbit HVS-T cell line (7710), even after 50 passages over a period of 2.5 years, remained totally IL2-dependent and molecular studies revealed L-DNA deletion of 42.5 kbp. The DNA in the 7710 cell was of covalently closed circular form and contained 15 copies of HVS genome. This is the only HVS line which is highly oncogenic and bears DNA deletions which are different than those of T cell lines of monkey origin.